This invention relates generally to bedding or seating products and, more particularly, to a spring core for a mattress.
Traditionally, spring cores for mattresses have consisted of a plurality of spaced parallel rows of helical coil springs mounted between border wires; coil springs adjacent the border wires being attached thereto via helical lacing wires, sheet metal clips or other connectors. The upper and lower end turns of adjacent coil springs are generally connected to each other by helical lacing wires. Coil springs are arranged in longitudinally extending columns and transversely extending rows. Padding and upholstery commonly are secured to opposed surfaces of the spring core, thereby resulting in what is known in the industry as a two-sided mattress for use on either side.
Recently, spring cores have been developed having only one border wire to which the end turns of the outermost coil springs are secured. After padding and/or other materials are placed over the upper surface of the spring core in which the border wire is located, an upholstered covering is sewn or secured around the spring core and cushioning materials, thereby creating what is known in the industry as a one-sided or single-sided mattress.
The upper and lower end turns of unknotted coil springs often are made with portions or legs which abut one another when coil springs are placed next to each other. Adjacent coil springs are connected to each other at their end turns with helical lacing wire. One leg of an end turn of a coil spring is set beside the opposite leg of an end turn of the adjacent coil spring. The side-by-side legs are laced together with helical lacing wire.
The greatest expense in manufacturing spring cores or assemblies is the cost of the raw material, the cost of the steel used to make the coil springs which are assembled together. For many years, the wire from which unknotted coil springs have been manufactured has a tensile strength no greater than 290,000 psi. This standard wire, otherwise known as AC&K (Automatic Coiling and Knotting) grade wire has a tensile strength on the order of 220,000 to 260,000 and is thicker, i.e., has a greater diameter, than high tensile strength wire, i.e., wire having a tensile strength greater than 265,000 psi. In order to achieve the same resiliency or bounce back, a coil spring made of standard gauge wire must have one half an additional turn when compared to a coil spring made of high tensile wire. In other words, the pitch of the coil springs made of high tensile wire may be greater as compared to coil springs made of standard wire. Coil springs made of high tensile strength wire also do not tend to set or permanently deform when placed under significant load for an extended period of time, i.e., during shipping. Therefore, there is a desire in the industry to make coil springs having unknotted end turns of high tensile strength wire because less wire is necessary to manufacture each coil spring.
U.S. Pat. No. 7,386,897 discloses a bedding or seating product having a spring core made of high tensile strength wire. However, each of the spring cores disclosed in this patent has at least one border wire having a round cross-sectional configuration.
The border wire of these and other known spring cores are often six-gauge, having a diameter of 0.191 inches. To make a border wire having the same beam strength, but made from a smaller diameter wire, say seven-gauge wire having a diameter of 0.176 inches, would save material and therefore reduce the end cost of the innerspring or spring core. In order to achieve the same beam strength, the seven-gauge border wire must be changed or shaped from a circular cross-section to a rectangular cross-section in accordance with the present invention. Thus, the present invention enables one to use a seven-gauge wire rather than a six-gauge wire in the border wire of the spring core and therefore, reduce wire cost without giving up any beam strength.
In order to achieve cost savings, it would be desirable to reduce the cross sectional area of the border wire of a spring core (by creating the border wire from a smaller diameter wire) while maintaining the same beam strength or increasing it. Therefore, making a spring core having at least one border wire with a rectangular cross-section would result in cost savings for the manufacturer of the spring core.